F. KRAFT-HUNTER AND E. W. HAWTHORNE 
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calibrations are made for large changes in baro- 
metric pressure when this occurs. Similarly in- 
trathoracic pressure is calibrated when desired. 
In the interim between comparison calibrations, 
D.C. voltage levels representative of the estab- 
lished calibration positions (high and low val- 
ues) are used to "tell" the computer where 0 
and 50 mm of mercury is or some similar set of 
values. The amplifiers and recording system are 
held fixed at one sensitivity and balance posi- 
tion during these times. In addition, every pre- 
caution is taken to have accurately calibrated 
recordings. 
Measurement of Diameter (Inductance 
Micrometry) 
Pairs of induction coils are used to measure 
diameters. One pair is placed internally across 
the diameter on the plane of the minor semiaxis 
of the left ventricle, and one pair is fixed to the 
external surface of the left ventricle in the same 
plane. Both of these are diagrammed in Figure 1. 
Inductance micrometry involves exciting one 
coil of each pair with an oscillating sine wave 
current at constant voltage and amperage and 
monitoring the amplified, rectified, and demodu- 
lated induced EMF from the opposite receiving 
coil of each pair. A linearized output is achieved 
through a specially designed circuit (Perry, 
Hinds, Hawthorne: A linearizing inductance 
micrometer for monitoring left ventricular in- 
ternal diameter). This permits calibration of 
the coils in terms of the amount of separation in 
centimeters versus voltage output. The recorded 
variations in D.C. voltage with changes in ven- 
tricular diameter is a linear and inverse func- 
tion of separation of the coils over a range of 
2.5 to 6 mm separation. 
Electrograms and Pacing Wires 
Pairs of teflon coated multistranded stainless 
steel wires are used for electrodes and pacing 
wires. Bipolar electrograms are recorded from 
the left atrium and left ventricle. 
Lead Beads 
Shown in Figure 1 is the placement of pairs 
of lead beads of different sizes. These are in- 
serted to a subendocardial position at the time 
of surgery. We use these opaque markers to 
study change in left ventricular length and di- 
ameter by biplane cineradiography. 
Pre and Post Operative Care 
All dogs are kept continuously under close 
medical supervision and appropriate antiar- 
rhythmic, antibotic, nutritional and other indi- 
cated therapy is continuously available to them. 
The dogs chosen for study are kept in excellent 
clinical condition. We do not use animals which 
have high temperatures, diarrhea, worms, 
pleural fluid, pericardial effusion and the like. 
No dog is seriously studied until at least two 
weeks post operative. All animals are given 
atropine, xylocaine, and a broad spectrum anti- 
biotic prophylactically during the first 48 hours 
after surgery. Antibiotics are continued until 
body temperature has returned to normal. We 
use the dog's instrumentation to observe the 
condition of these animals, through continuous 
monitoring, until they recover from anesthesia 
for the first 5 hours after surgery and for 12 
days post operative. Since all the dogs collect 
fluid during this period, the chest is drained of 
all collected fluid. After the first 12 post opera- 
tive days no further fluid sems to collect in the 
chest of most dogs. Those that still have effu- 
sion at this time usually do not recover satisfac- 
torily. The chest drainage tube is removed at 
this time. Chest x-rays are made at infrequent 
intervals in all dogs. 
Data Acquisition and Reduction 
With the transducers in place, as described 
above, their voltage outputs are cabled to the 
appropriate carrier, D.C. coupling, high and 
low gain amplifiers. The amplified and condi- 
tioned signals are then distributed, as required, 
by patch panel connections to (a) recording 
system for permanent graphic chart of analog 
signals; (b) to a multichannel scope for moni- 
toring and display; (c) to an analog magnetic 
tape recorder for storage of all data signals; 
(d) to an A to D converter for input to the com- 
puter. In this mode, data can be sampled on 
line, as well as retrieved from magnetic tape for 
study and selection of portions for detailed 
processing by the computer. The data is proc- 
